It is known to separate such a mixture in order to produce carbon monoxide and hydrogen by a methane scrubbing process as described in Linde Reports on Science and Technology, “Progress in H2/CO Low-Temperature Separation” by Berninger, 44/1988 and in “A New Generation of Cryogenic H2/CO Separation Processes Successfully in Operation at Two Different Antwerp Sites” by Belloni, International Symposium on Gas Separation Technology, 1989.
Other documents that describe methane scrubbing processes include: EP-A-0928937, U.S. Pat. No. 4,478,621, Tieftemperaturtechnik, page 418.
The carbon monoxide that results from H2/CO cold boxes entrains with it a significant fraction of nitrogen present in the feed gas. This phenomenon is linked to the difficulty in separating the two components CO and N2, their bubble points being very close. Nevertheless, depending on the use which is made of the CO downstream of the cold box, it sometimes proves necessary to reduce its nitrogen content before exporting it.
In order to do this, recourse has conventionally been made to the installation in the cold box of a column known as a denitrogenation column, the role of which is to produce, as bottoms, carbon monoxide at the required purity. At the top of the column, a nitrogen purge is recovered that contains a fraction of CO. The denitrogenation column is installed either upstream, or downstream of the CO/CH4 separation column.
One of the existing processes described in U.S. Pat. No. 4,478,621 comprises a denitrogenation column equipped with an overhead condenser. The refrigerant for the overhead condenser of the denitrogenation column is liquid CO, the pressure of which is close to atmospheric pressure. At this pressure level, the vaporization temperature of the CO is too low to cool the feed gas at the inlet of the methane scrubbing column: the methane would risk freezing. In order to cool the feed gas, the process thus provides a vaporization of CO at a higher pressure level.